1. Field of the Invention
The present invention relates to a flat panel X-ray detector used in a medical X-ray diagnostic apparatus.
2. Description of the Related Art
In recent years, preparation of data base on the medical data on a patient is being promoted in a medical field in order to perform the medical treatment promptly and appropriately. It should be noted in this connection that the patient utilizes in general a plurality of medical organizations. Therefore, if there is no data prepared in another medical organization, there is a possibility that an appropriate medical treatment is not performed.
The data base preparation is also required in respect of the image data of the X-ray photography, and it is hoped from this point that a digital system be developed in respect of the X-ray photograph. It was customary in the past to use a silver halide film in the medical X-ray diagnostic apparatus. For employing a digital system in the medical X-ray diagnostic apparatus, it was necessary to develop the photograph film image and scan again the developed film image with a scanner, which was laborious and time-consuming.
In recent years, a system of directly converting the image into digital data has been realized by using a CCD camera sized about one inch. However, in photographing, for example, a chest, a region of about 40 cm×40 cm is photographed, making it necessary to use an optical apparatus for collecting light, leading to the problem that the apparatus is rendered bulky.
As a system for overcoming the above-noted problems inherent in the two systems described above, a flat panel X-ray detector using an amorphous silicon thin film transistor (a-Si TFT) is proposed in, for example, U.S. Pat. No. 4,689,487.
The flat panel X-ray detector proposed in the U.S. patent quoted above comprises a large number of pixels each formed of a-Si TFT, a photoconductive film and a pixel capacitor. These pixels are arranged to form an array (hereinafter referred to as a TFT array), the row of the array consisting of hundreds of to thousands of pixels and the column of the array also consisting of hundreds of to thousands of pixels.
A bias voltage is applied from a power source to the photoconductive film. The a-Si TFT is connected to a signal line and to a scanning line and is subjected to an on-off control by a scanning line driving circuit. The terminal of the signal line is connected to an amplifier for the signal detection. If X-ray light is incident on the X-ray detector, an electric current flows into the photoconductive film so as to permit the electric charge to be stored in the pixel capacitor. If the scanning line is driven by the scanning line driving circuit, all the TFTs connected to a single scanning line are turned on, with the result that the scanning line permits the stored electric charge noted above to be transferred toward the amplifier. Then, the change-over switch is changed over so as to permit the electric charge for each pixel to be sequentially amplified in the amplifier and, then, the amplified electric charge is displayed in a display device.
The system described above makes it possible to subject the output signal of the amplifier to an A/D conversion so as to obtain a digital image. The particular system is equal to a flat panel X-ray detector based on an indirect conversion system in which the incident X-ray is converted by a phosphor into a visible light and, then, the visible light is further converted into an electric charge by the photoconductive film. Also known is a flat panel X-ray detector of a direct conversion system that does not include the phosphor, in which the X-ray is converted directly into an electric charge by a photoconductive film.
In the flat panel X-ray detector of this type, it is necessary for the signal charge generated by the X-ray to be migrated promptly so as to reach the pixel electrode and to be stored in the storage capacitor section. However, where the signal charge remains inside the photosensitive film, a defective image is generated by the presence of an afterimage and the decrease of the resolution. In many cases, the defective image is generated by the situation that a signal charge remains within the photosensitive film so as to affect the flow of a signal charge that is newly generated by an X-ray. Also, where there are many defects area, an electric charge flows through the defects so as to give rise to the problem that the dark current is increased.
It is known to the art that PbI2 is used for forming the X-ray-electric-charge converting film. The PbI2 material is expected to exhibit excellent characteristics. However, PbI2 is insufficient in its crystallinity when a thin film is actually formed by using PbI2, leading to various problems such as the afterimage, the resolution deterioration and the large dark current as pointed out above. Such being the situation, a PbI2 film exhibiting sufficient characteristics has not yet been achieved (see, for example, “R. A. Street et al., SPIE Vol. 3659, p. 36, 1999”).
Since it is known to the art to form a blocking layer made of AsSe in contact with an X-ray-electric-charge converting film made of Se, it is conceivable to form a blocking layer of a carrier in contact with an X-ray-electric-charge converting film made of a metal halide in order to overcome the problem that the dark current is large. However, it is difficult to form a blocking layer that is low in its reactivity with a metal halide, with the result that it is difficult to form a blocking layer having an excellent interface property between the blocking layer and the X-ray-electric-charge converting film. And, a blocking layer effective for an X-ray-electric-charge converting film made of a metal halide was unknown in this technical field.
As described above, in the conventional flat panel X-ray detector, it was impossible to form a blocking layer, which was low in its reactivity with the material forming the X-ray-electric-charge converting film and which was effective for forming an interface having an excellent property between the blocking layer and the X-ray-electric-charge converting film, with the result that it was difficult to lower sufficiently the dark current of the flat panel X-ray detector.